Generally, the same devices can be used for both multiplexing and demultiplexing operations. The difference is merely the result of opposite directions of light travel through the devices. Multiplexers route signals of different wavelengths (also referred to as channels) traveling in multiple optical pathways into a single pathway. Demultiplexers route different wavelength signals from a single pathway into respective multiple pathways.
A variety of techniques are used within these devices to distinguish the different wavelength signals. One such technique involves varying optical path lengths of intermediate pathways between the single and multiple pathways to angularly separate different wavelength signals. Waveguides of varying length are arranged in a lateral progression to relatively vary phases of the different wavelength signals transverse to their direction of propagation. Generally, the path length differences are an integer multiple of a central wavelength signal, whose wavefront is not affected by the different distances of travel; but the remaining wavelength signals progressively vary in inclination as a function of their wavelength. For example, the wavefront of the wavelength that differs most from the central wavelength is also the most inclined.
In the demultiplexing direction, the different wavelength signals enter the different length intermediate pathways as parallel wavefronts and exit the intermediate pathways as relatively inclined wavefronts. The entrance and exit are reversed for multiplexing operations. Focusing is used to convert the angular separation between wavefronts into a linear separation coincident with a lateral array of the multiple pathways.
Each of the different wavelength signals entering the devices exhibits a mode field that can be defined by a pattern of radiation in a plane transverse to the direction of propagation. Ordinarily, the pattern follows a Gaussian-type distribution. The intermediate pathways individually convey different sections of the mode fields of each signal; but collectively, the intermediate pathways preserve the overall distribution of energy in the original mode fields (i.e., the peak intensities in the intermediate pathways follow a pattern that matches the distribution of energy in the original mode fields).
However, such distributions are not well suited for efficiently coupling the inclined wavefronts to the laterally arrayed multiple pathways. The different wavelength signals having inclined wavefronts are also effectively inclined to the direction of propagation and focus at positions that are correspondingly offset from the focus position of a non-inclined wavefront. As a result, transmission efficiency tends to decrease with increasing amounts of wavefront inclination. That is, the central wavelength signal couples most efficiently; but other wavelength signals exhibit greater losses, especially those wavelength signals most remote from the central wavelength signal.